This invention relates to a radio communication apparatus which receives a multilevel modulated signal. The radio communication apparatus may be a radio paging receiver, a transceiver, or the like although description will be mainly made in connection with the radio paging receiver.
A conventional radio paging receiver of the type described, generally comprises demodulator which demodulates the multilevel modulated signal into a time sequence of multilevel symbols having symbol levels. A signal converting section converts the time sequence of multilevel symbols into a binary or digital signal of binary or digital values by judging said symbol levels on the basis of judgement levels.
The symbol levels of the time sequence of multilevel symbols are liable to a level variation due to a signal variation of the multilevel modulated signal. In order to compensate for the level variation of the symbol levels, the signal converting section varies the judgement levels in accordance with the level variation of the symbol levels. Alternatively, the signal converting section subtracts the level variation from the symbol levels with the judgement levels kept in fixed values.
Such signal converting sections can correctly judge the symbol levels when the level variation of the symbol levels is less than half of a judgement level difference between two adjacent ones of the judgement levels. However, the signal converting sections unavoidably misjudge the symbol levels when the level variation is not less than half of the judgement level difference. Such misjudgement is maintained for a succeeding part of the multilevel modulated signal without recovery from a misjudgement state.